This invention relates to semiconductor devices such as an insulated-gate bipolar transistor (IGBT), and concerns more specifically how to enable such semiconductor devices to withstand higher voltages, particularly reverse voltages, than heretofore.
The IGBT such as that disclosed in Japanese Unexamined Patent Publication No. 6-69509 lends itself to use as a solid-state switch capable of self-extinction of arc. It can be turned on or off at any point on a sinusoidal waveform. Typical applications of the IGBT include, therefore, the AC switching of the matrix converter for induction motor speed control and the switching of AC power supplies.
Being subjected to both positive and negative half cycles of an alternating wave, the IGBT when used as an AC switch must be capable of withstanding sufficiently high voltages in both forward and reverse directions. The IGBT is generally weaker to reverse, than to forward, voltage. For AC switching, therefore, two IGBTs have been connected in parallel with each other for separately handling positive and negative currents, and a reverse-blocking diode has so far been connected in series with each IGBT in order to circumvent the noted weakness thereof. Use of the reverse-blocking diodes is of course objectionable for the conduction losses introduced thereby, which indeed have caused a significant diminution of the efficiency of the device incorporating the AC switch.
Attempts have been made for making the IGBT strong enough against reverse voltages to do without the reverse-blocking diodes. One such attempt is based upon the discovery that the insufficient strength of the conventional IGBT against the reverse voltage is due largely to current leakage from the side surfaces of each IGBT chip which are created upon dicing of the semiconductor wafer into individual chips with a diamond cutter. A conventional remedy for this deficiency was the provision of an antileakage layer on the side surfaces of the IGBT chip. The antileakage layer is formed by diffusion of impurities into part of the IGBT chip substrate, causing that part to gain the same conductivity type (e.g., p+) as the collector region of the IGBT. The resulting antileakage layer is joined directly to the collector region. Surrounding the base (drift) region in combination with the collector region, the antileakage layer serves to prevent the peripheries of the pn junction between the collector and base regions from being exposed at the side surfaces of the IGBT chip. Current leakage from the side surfaces was thus reduced, and the IGBT became stronger against reverse voltages.
However, this conventional remedy proved to possess its own weakness when applied to IGBTs designed to withstand as high a voltage as, say, 1200 volts. Such an extremely high voltage IGBT must have a base region as thick as 130 micrometers and necessitates the creation of an antileakage layer of matching depth. The creation of an antileakage layer to such a depth by diffusion was not only time-consuming but prone to give rise to crystal defects within the semiconductor substrate. Additionally, as the antileakage layer became deeper, so it also grew wider as a result of lateral diffusion that unavoidably accompanied the desired diffusion in the depth direction of the substrate, making the IGBT chip greater in overall size.
The instant applicant is aware that it has been known to create trenches between the individual devices fabricated on the semiconductor wafer and to fill the trenches with an insulator. These insulator-filled trenches are se not conducive to making the peripheries of the pn junction more resistive to voltages.
The difficulties discussed above are not limited to IGBTs. Similar problems have been encountered with other semiconductor devices such as bidirectional thyristors.